An inkjet printer typically comprises one or more print heads, each of which has a number of nozzles (for example, 100 nozzles) through which ink droplets can be ejected to produce a mark on a substrate at a desired location. The throughput of an inkjet printer relates to the number of nozzles that can be operated simultaneously. In order to increase the throughput, it is known to increase the number of print heads in a printer. Large numbers of print heads, for example tens or hundreds, can be arranged in an assembly known as an array. This is particularly common in large-format industrial printers. An ‘array’ as used herein means an assembly comprising a plurality of substantially identical print heads arranged to print using the same type of ink. The print heads may be arranged in line or over an area (such as a rectangle).
It is known to deliver ink to each print head in an array via individual tubes. It is desirable to circulate ink through the print head, and typically two tubes are required for each print head. While this arrangement is satisfactory for small arrays, of perhaps 20 print heads, in larger arrays, which may comprise over 150 print heads, the number of tubes (300) it is necessary to connect is prohibitively difficult and complicated. In addition, ink leakage from such tubes is common, and locating the source of a leak among 40 tubes, let alone among 300 tubes, is very difficult. Furthermore, large numbers of tubes result in large pressure length losses inside the ink supply chain.
For maximum printer throughput, it is desirable for the length of an array to equal the length or width of the substrate to be printed. In wide-format printers, such an array might comprise over 300 print heads, and exceed two meters, or even five meters, in length. Ink supply to such an array using the above system would require over 600 tubes and, given the problem of ink leakage, is not commercially viable. We have found it desirable to provide an alternative way of supplying ink to print heads in an array to enable the construction of larger arrays, and to avoid the problem of ink leakage.
During printing, the temperature of the array may vary, as the print heads and associated electronics warm up in use. Temperature variation over the course of a print run is undesirable, as the viscosity of most inks varies with temperature. That variation affects the amount of ink ejected from the nozzles of the print head, and will, for example, cause variation in the colour or intensity of different versions of the same image printed at different temperatures. In addition, some inks, such as ultraviolet inks, only obtain the correct printing properties in a particular, elevated, temperature range, and may not print correctly, or at all, outside that range. Furthermore, the dimensions of the array itself may vary with temperature, adversely affecting the image. We have found it desirable to stabilise the temperature of an array.
An additional problem with existing arrays is known as ‘ink surge’ or ‘water hammer’. That is, when a large number of print head nozzles, for example the nozzles of all 300 or 400 print heads, begin to operate simultaneously, they consume a large amount of ink, forming a zone of lower pressure close to the nozzles and resulting in a delay in ink ejection from the nozzles. Similarly, when a large number of nozzles cease operating simultaneously, excessive pressure builds up in the ink supply system. We have found it desirable to alleviate this effect, and stabilise the ink supply to the array.